Honestly, I think very small cases only work well when you have a specific focus and know exactly what you want – which usually means you have some experience with modular. Otherwise, you end up sacrificing a lot of features, playability and fun to meet the need for compactness.
(In a way it’s like an aquarium: it seems at first like a tiny 2.5 gallon tank is the way to go, but you can’t put much in there and it’s actually much harder to keep the water parameters stable and the fish healthy than something in the 20-30 gallon range.)
To start off small and cheap, a semi-modular is really hard to beat. An Erica Pico System III, an 0-Coast or a Neutron pack in a lot more functionality than you’re going to get at that price and size with individual modules, and they’re already set up to be a fully usable synth voice.
Otherwise I wouldn’t start with less than a 104HP case, or something like the Intellijel 7U. I tried to start with a DIY 74HP box, and though I had a good start for the first few modules, I made some poor compromises soon afterward in the name of conserving HP.